Gamma ray measurement of earth formation properties using a position sensitive scintillation detector

ABSTRACT

A well logging system for measuring the radial density distribution of earth formations in the vicinity of a well borehole is provided. The system utilizes a single position sensitive gamma ray detector capable of deriving formation bulk density using the gamma-gamma scattering technique at different radial distances from the well borehole.

BACKGROUND OF THE INVENTION

This invention relates to gamma ray scattering or gamma-gamma densitywell logging techniques and, more particularly, to such techniques forderiving a compensated formation bulk density by the use of a positionsensitive scintillation detector.

It has become fairly common practice in the art of well logging to logthe earth formations in the vicinity of a well borehole which is eithercased or uncased with a gamma ray density instrument. Such an instrumentcomprises a source of gamma rays such as cesium 137 which are collimatedand directed outwardly into the formation from the well logging sonde ortool which is lowered into the borehole on an electrical wireline. Thegamma rays are scattered from the electrons of elements comprising theearth formations in the vicinity of the borehole. A separate gamma raydetector longitudinally spaced from the gamma ray source then is used tomeasure the intensity of scattered gamma rays from the materialssurrounding the well borehole back into the instrument. More than onesuch detector may be used in order to provide a compensated densitymeasurement which is compensated for the presence of mudcake or casingthickness intervening between the gamma ray source and the detector ordetectors. An example of such a system is given in U.S. Pat. No.4,297,575 which is assigned to the assignee of the present invention.

In the well logging system described in the aforementioned patent, thegamma ray source is positioned below two gamma ray detectors which arelongitudinally spaced at different distances from the source. Gamma raysfrom the source are directed by a collimator into the earth formationsin the vicinity of the wellbore. Scattered gamma rays returning fromthese formations are directed by collimators to the two detectors. Inthis system, the near spaced detector to the source is a Geiger-Muellertype counting tube and the far spaced detector is a sodium iodidethorium activated scintillation detector having a photomultiplier tubeoptically coupled thereto. The thickness of casing or mudcake may bedetermined by appropriate computations based on predeterminedrelationships existing between the count rate of scattered gamma rays inthe short spaced detector to those in the long spaced detector. Thecount rate in the short spaced detector is much more influenced bymaterial closer to the well borehole than that of the long spaceddetector. By appropriately combining the count rates in the twodetectors, which are spaced a known distance apart, and the use of apredetermined calibrated relationship between the formation bulk densityand the count rates at the two detectors the formation density may becomputed independently of the effects of the mudcake or casingintervening the distance between the scattered gamma rays from thesource and the two detectors.

Certain problems are encountered in a system of the type described inthe above referenced U.S. Patent. These concern the sensitivity orrelative sensitivity to gamma rays of the two different types ofdetectors and the spacing distance used to separate the detectors beingrelated geometrically to the depth of investigation of the instrumentinto the earth formation surrounding the well borehole. It would behighly desirable to have a multiplicity of detectors located above thegamma ray source so that the formation bulk density at different radialdistances from the well borehole could be investigated and a radialdensity cross-section of the earth formation in the vicinity of theborehole plotted as a function of borehole depth. It is a feature of thepresent invention that a single unique position sensitive scintillationdetector is utilized together with a gamma ray source to providemeasurements of formation bulk density at different radial distancesfrom the well borehole and to compensate the measured formationdensities for the effects of mudcake or intervening casing thickness andcement thicknesses between the well borehole and the surrounding earthformations. It is another feature of the present invention to be able toderive formation bulk density measurements at different radial distancesfrom a well borehole and to plot such formation densities as a functionof borehole depth. Yet another feature of the present inventioncomprises the ability to measure the scattered gamma ray energydependent response from earth formations adjacent a well borehole atdifferent radial distances from the well borehole with a single positionsensitive radiation detector.

BRIEF DESCRIPTION OF THE INVENTION

In the gamma ray density logging system of the present invention, adownhole sonde is provided at its lower end with a gamma ray sourcewhich is collimated to direct gamma rays from the source outwardly intothe earth formations in the vicinity of the well borehole. A singlenovel position sensitive radiation detector is longitudinally spacedabove the gamma ray source in the instrument and is used to detectscattered gamma rays coming from different radial distances from theborehole into the formation. Plural collimators are used to direct gammarays scattered from the earth formations to the single positionsensitive gamma ray detector of the present invention. Dualphotomultiplier tubes located at opposite ends of a cylindricalscintillation crystal are utilized to determine at what distance orposition from a reference end of the crystal the gamma ray has impingedon the scintillation detector crystal. This information in turn may berelated to gamma rays which have been scattered from different radialdistances away from the well borehole. Density measurements of theformation bulk density based on different radial distances from theborehole may thus be determined. Two separate types of positionsensitive radiation detectors for use in such a density logging systemare disclosed herein. Either of these types of position sensitiveradiation detectors may utilize dual photomultipliers positioned atopposite ends of a cylindrical or elongated scintillation crystaldetector.

The invention may best be understood by reference to the followingdetailed description thereof when taken in conjunction with theaccompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing a well logging system inaccordance with the concepts of the present invention in a wellborehole.

FIG. 2 is a schematic illustration showing a position sensitive gammaradiation detector of one type used in the system of the presentinvention.

FIG. 3 is a schematic diagram illustrating the position sensitiveradiation detector of FIG. 2 in position against the wall of a wellborehole and showing a gamma ray source and collimators used in thesystem.

FIG. 4 is a schematic diagram illustrating the geometry of spreadingwavefronts of light in a cylindrical scintillation crystal used in onetype of detector in accordance with the present invention.

FIG. 5 is a schematic diagram illustrating a second type of positionsensitive scintillation detector which may be utilized in a systemaccording to the concepts of the present invention.

FIG. 6 is a graphical representation of the output voltages ratio of adetector such as shown in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

Details of the techniques for determining compensated bulk density ofearth formations the vicinity of a well borehole are given in theaforementioned U.S. Pat. No. 4,297,575. A discussion of the theory ofscattered gamma ray or gamma-gamma density logging is included in thisU.S. Patent and will not be dwelled upon in detail herein. It willsuffice to say that the same cross-plotting techniques for determiningformation bulk density and which utilize scattered gamma rays fromdifferent radial distances into earth formations surrounding a wellborehole can be used with the system of the present invention. By theuse of a unique position sensitive radiation detector in the presentsystem, a single longitudinal crystal having a length L may besubstituted in place of the separate plural detectors used to sensescattered gamma rays from different radial distances in the formationsin the aforementioned U.S. Patent. The relative location or positionalong the length L of a detector crystal at which a particular gamma rayentering the crystal occurs may be determined with the positionsensitive detectors of the present invention. Thus different radialdepths of investigation may be investigated with this single detector.Using one type of detector the length distribution along the detector ofgammas entering the detector may be determined continuously over theentire length of the detector. In a second type of detector which isalso described herein, the position of gamma rays entering the detectorcrystal is divided into separate longitudinal bins or positions havingdifferent longitudinal distances from the gamma ray source. Thus using asingle scintillation detecting crystal and a pair of photomultipliers ateach end of the crystal the location into which partition or bin aparticular gamma ray has been detected in the crystal may be determined.

The result of the use of such position sensitive scintillation detectorsin a gamma gamma density measuring system in accordance with theconcepts of the present invention is to provide measurements ofscattered gamma rays occurring at a plurality of discrete or continuumof different radial distances from the well borehole. This can beutilized to provide a density cross section of the earth formationsgoing away from the wall of the well borehole deeper into the formationforming the wall of the borehole.

By the use of a single position sensitive radiation detector inaccordance with the concepts of the invention, the use of a multiplicityof individual detectors is avoided thereby simplifying the well loggingsystem immensely in terms of the required electronic circuitry. Valuableinformation concerning the density cross-section at different radialdistances from the well borehole is thus provided in accordance with theconcepts of the present system.

Referring initially to FIG. 1, a well logging system in accordance withthe present invention is illustrated schematically in a well borehole. Awell borehole 10 filled with a borehole fluid 12 penetrates earthformations 11 which have different density characteristics. A fluidtight well logging sonde 13 is suspended in the borehole 10 by a typicalmulticonductor (or single conductor) armored well logging cable 14. Thesonde 13 is eccentered or urged against one wall of the borehole 10 by abackup arm or skid member 15. The sonde is provided near the lower endthereof with a gamma ray source 16 which is collimated by an aperture ina shielding material 17 such as tungsten surrounding the source 16 toemit gamma rays in a preferred direction into the earth formations 11surrounding the borehole. A position sensitive gamma ray detectorcomprising photomultipliers 20 and 21 and a scintillation crystal 19 islongitudinally spaced in the downhole sonde above the gamma ray source16. The photomultipliers 20 and 21 are connected to circuitry 18 and toa telemetry unit 22 in a manner which will be described in more detailsubsequently for transmission of signals to the surface of the earth viaconductors of the logging cable 14.

At the surface the logging cable 14 passes over a sheave wheel 23 whichis electrically or mechanically coupled, as indicated by dashed line 24,to a recorder 27 which moves a record medium 28 as a function of theborehole depth of the downhole measuring instrument or sonde 13. Countrate information from the photomultipliers downhole position sensitivedetector 19, 20 and 21 is supplied by conductors of the logging cable 14to a surface density computer 26 which utilizes the count rateinformation at different longitudinal distances along the detectorcrystal 19 from the source 16 to determine the formation bulk density asa function of radial distance from the wall of the well borehole 10 ofthe earth formation materials surrounding the well borehole. Formationbulk density at different radial distances ρ₁, ρ₂, ρ₃ through ρ_(n) areplotted on the record medium 28 by the recorder 27 as a function of theborehole depth of the downhole sonde 13.

Power for the operation of the downhole system is provided by a surfacepower supply 25 which is coupled to conductors of the armored loggingcable 14 in a conventional manner.

Referring now to FIG. 2, one type of position sensitive scintillationdetector which may be used in the gamma gamma density logging system ofthe present invention is illustrated in more detail but stillschematically. A single cylindrical shaped scintillation crystal 32,which may be of the thallium activated sodium iodide type, isillustrated. The scintillation crystal 32 is surrounded by a lightabsorbent material 33 along the entire length thereof. This is in directconstrast to a typical scintillation detector crystal which would betypically be surrounded by a light reflective medium. Located at eachopposite end of the cylindrical scintillation crystal 32 arephotomultipliers 30 and 31. The photomultipliers each sense lightflashes caused in the scintillation crystal by gamma rays impingingthereon and produce output voltage pulses whose voltage level isproportional to the intensity thereof. Each such light flash willprovide an output voltage signal from each of the two photomultipliers30 and 31. The output signal amplitude from photomultiplier 30 islabelled signal "A" in FIG. 2 and the output signal amplitude fromphotomultiplier 31 is labelled signal "B" in FIG. 2. A reference end ofthe crystal 32 is taken as zero at the end on which photomultiplier 30is attached and optically coupled thereto, and an output signal location"L" is indicated at the opposite end of the scintillation crystal towhich photomultiplier 31 is attached. It may be shown that a gamma rayimpinging on the crystal at an arbitrary position or location "X" ofFIG. 2 along its length may be located (i.e. the distance "X" between 0and L may be determined) from the expression of equation 1. ##EQU1##Equation 1 is valid where the length of the crystal is large relative toits diameter.

In equation 1 "X" represents the position of the scintillation eventalong the crystal, L represents the length of the crystal, A theamplitude of the signal received at end A, and B the amplitude of thesignal received at end B. The expression of equation 1 is based on thescintillation crystal being surrounded by a light absorbing medium andwherein geometrically spherical spreading of the light from a pointscintillation event occurring in the crystal is used. An inverse squaredattenuation function due to sperical spreading is assumed in thederivation of this expression.

Referring now to FIG. 4, the geometry of a more realistic situation isillustrated schematically. In the diagram of FIG. 4 it is not assumedthat the length of the cylindrical crystal is large relative to itsdiameter. The magnitude of the signal received at the end of the crystalin this instance includes the relationship between proximity of thegamma ray event in the crystal to the end of the crystal and the solidangle covered by the crystal end and viewed by the photomultiplier. Agamma ray event occurring as a point source at "X" in the crystal ofFIG. 4 will have distributed its scintillated light over a sphere radius"R" by the time the light reaches the end "A". The surface area of thissphere is 4πR². The surface area of the end of the cone formed by theend of the crystal and the sphere is 2πR² (1-X/R). Therefore the portionof light that reaches the end of the crystal is 1/2-X/2R. ##EQU2## itcan be shown that: ##EQU3## where E is the energy (amplitude) of theoriginal gamma ray event in the crystal at position X from the referenceend. By solving simultaneous equations 3 and 4 just given, X and E maybe determined as a function of A, B, H, and L. Here A and B are the samenomenclature as previously described, and H and L refer to the radius ofthe crystal and the length of the crystal as illustrated in FIG. 4.

Circuitry 18 of the system FIG. 1 quantitatively digitizes the amplitudeof the output signals from the two photomultipliers placed at oppositeends of the crystal and transmits this information via telemetry circuit22 to the surface density computer 26. The density computer using therelationship of equation 1, or alternatively, solving simultaneousequations 3 and 4 may then be used to compute the distance X along thescintillation detector crystal at which the scintillation event occursand energy E of the event. Thus, the distribution or position of thegamma rays impinging upon the crystal may be determined for eachscintillation event which occurs in the crystal. The density computer 26may be utilized to combine these according to predeterminedrelationships between the count rates at different distances along thecrystal in the manner described in the aforementioned patent using thetypical "spine and ribs" crossplots to provide compensated densitymeasurements at different radial distances away from the well bore.

Referring now to FIG. 3, a position sensitive detector of the type shownin FIG. 2 in position along the wall of a well borehole is illustratedschematically. Note that in FIG. 3, R refers to the radial distance fromthe center line or axis, Z, of the detector into the earth formationsurrounding the well bore. The scintillation crystal 43 is provided atoppoiste ends thereof with photomultipliers 41 and 42 which may be ofthe type previously described. A gamma ray source labelled γ in FIG. 3and which may comprise a cesium 137 source is collimated to distributegmma rays in a preferential directional into the earth formationssurrounding the borehole. A mudcake layer and a mud filtrate penetrationlayer in virgin formation is illustrated in FIG. 3. A shielding materialsuch as tungsten (shaded) is provided with collimation slots and allowsscattered gamma rays from different radial distances R into the earthformation to be scattered back onto different longitudinal positionsalong the scintillation crystal 43 of the position sensitive radiationdetector. Count rates at different longitudinal locations along thecrystal may be used in the manner previously described with respect tothe aforementioned U.S. Patent to determine formation bulk density atthe different radial distances into the earth formation corresponding tothe mudcake, the mud filtrate invaded layer and the virgin formation.Thus, the thicknesses of the mudcake and the filtrate invaded zone maybe determined using the techniques of the present invention.

Rather than using a clear, completely optically transparentscintillation crystal surrounded by a light absorbent material in thedetector type shown in FIG. 2 and relying on attenuation by sphericalspreading of a wavefront, the same principle for determining positionalong the crystal could be used in another embodiment of this type ofdetector. A light abosrbent or "cloudy" crystal having a knownabsorption characteristic per unit of length could be used. Lightabsorption in the crystal could be provided by doping the thalliumactivated sodium iodide crystal during its manufacture with a dye havingthe desired absorptive properties as a function of optical path length.This tailored absorptive property could be thought of as "forcedabsorption" as opposed to "natural absorption" which occurs in aspreading spherical wavefront. Appropriate expressions for X and Eanalagous to Equations 3 and 4 could then be derived based on thisabsorptive characteristic plus spherical spreading if the crystal weresurrounded by on light absorptive medium or by the forced absorptivelight characertistic alone if the crystal were surrounded by anefficient light reflective medium. This type of development can lead toa second, different type of position sensitive detector for use in thesystem of the present invention which is illustrated in FIG. 5.

Referring now to FIG. 5, a different type of position sensitiveradiation detector which may be utilized in a gamma ray density loggingsystem according to the concepts of the present invention isillustrated. In the detector of FIG. 5, a cylindrical scintillationdetector comprising plural cylindrical crystals is divided into binslabelled bin 1, bin 2, bin 3,--to bin N along the length thereof by aplurality of neutral density filters as shown. This array of separatescintillation crystals, which may be of the optically transparentthallium activated sodium iodide type, is provided at each end thereofwith photomultiplier tube detectors (labelled PMT) and having outputvoltage signals A and B. The position along the length of thescintillation detector array of a gamma ray event may be determined bytaking the ratio of the amplitude of the signals from the twophotomultipliers. In this case the ratio response will appear as a stairstep function by the action of the neutral density filters on the lightgenerated within the scintillation crystal.

In the detector shown in FIG. 5, it is not necessary to surround thescintillation crystals with a light absorbant medium. In this type ofdetector the scintillation crystal may be surrounded by a conventionallight reflecting medium. Thus, a high light retention efficiency maybeachieved by this type of detector with the loss of light intensityresolution being limited to that provided by the absorption of light bythe neutral density filters. A gamma ray scintillation event occurringin a particular bin along the length of the scintillation crystal willbe characterized by a particular ratio of signal A/B output from thephotomultipliers. The particular characteristic step on the stair stepfunction (shown in FIG. 6) relating the ratio of signal A to signal Bwill characterize which particular bin (from 1 to N) of thescintillation crystal at which the scintillation event occurs. It willalso be noted in the detector of FIG. 5 that there is no constraintplaced on having the size of the different bins formed by the neutrondensity filters be the same. These bins may be whatever size is desiredand are defined only by the absorptive action or forced absorptivefunction of the neutral density filters. If the absorption function ofeach filter is known then the original amplitude of the scintillationevent may be reconstructed in a straightforward manner from theindividual PMT output of either photomultiplier, thus preserving theenergy E information of the original gamma ray event.

The stair step function relating the distance along the scintillationcrystal to the amplitude ratio A/B is illustrated in FIG. 6. In derivingthis function, it is assumed that the material within a bin does notsignificantly absorb light and that the surrounding reflecting surfacearound the cylindrical crystal is very efficient. Thus, the onlyabsorption in the light reaching either of the photomultiplier tubes isthat which occurs because of the absorption of the neutral densityfilters.

The foregoing descriptions may make other alternative embodimentsaccording to the concepts of the inventions apparent to those skilled inthe art. It is the aim of the appended claims to cover all such changesand modifications as fall within the true spirit and scope of theinvention.

I claim:
 1. A system for measuring properties of earth formations in thevicinity of a well borehole at a plurality of different radial distancesfrom the borehole, comprising:a fluid tight hollow body member sized andadapted for passage through a well borehole and housing therein; asource of gamma rays and means for directing gamma rays from said sourceoutwardly from said body member into earth formations in the vicinity ofthe borehole; and a position sensitive scintillation detector fordetecting gamma rays scattered back into said body member from the earthformation in the vicinity of the borehole, means for collimating saidscattered gamma rays onto said detector, said detector comprising ascintillation crystal longitudinally spaced from said gamma ray sourceand having a longitudinal length L and two opposite ends and havingphotomultiplier tubes optically coupled to said opposite ends forproviding output voltage signals having voltage amplitudes A and Brepresentative of the intensity of scintillation events occurring insaid crystal and impinging at said opposite ends thereof and means forcombining said output voltage signals A and B according to apredetermined relationship to derive the position along said length L ofeach of said scintillation events in said crystal, thereby providingmeasurements of the gamma ray scattering properties of the earthformations at different radial distances from the borehole.
 2. Thesystem of claim 1 wherein said scintillation crystal is generally of aright circular cylinder shape.
 3. The system of claim 1 wherein saidscintillation crystal is essentially optically transparent and issurrounded on the surface thereof by a light absorptive medium.
 4. Thesystem of claim 1 wherein said scintillation crystal has a predeterminedabsorptive function for light as a function of optical path length andis surrounded by a light absorptive medium.
 5. The system of claim 1wherein the earth formation property to be measured is the formationbulk density.
 6. The system of claim 5 wherein said formation bulkdensity is compensated for material intervening said gamma ray sourceand said formation by combining bulk density measurements attributableto different radial distances from the borehole according to apredetermined functional relationship.
 7. The system of claim 1 andfurther including means for deriving from said output voltage signals ameasure of the energy E of the gamma rays causing said scintillationevents.
 8. The system of claim 1 and further including means forrecording as a function of borehole depth and radial distance from theborehole at a given depth, the earth formation properties beingmeasured.
 9. A position sensitive scintillation detector for use in aborehole measurement system for measuring properties of earth formationsat different radial distances from a well borehole, said systemincluding a gamma ray source, means for directing gamma rays from saidsource outwardly into the earth formation and means for collimatinggamma rays scattered by the earth formation onto said position sensitivescintillation detector said detector comprising:a scintillation crystalhaving a generally elongated shape and having a length L and having twoopposite ends; photomultiplier means optically coupled to said oppositeends for providing output signals A and B representative of theintensity of scintillation events occuring in said crystal and impingingat said opposite ends thereof; and means for combining said outputsignals A and B according to a predetermined relationship to derive theposition along said length L of each of said scintillation events insaid crystal.
 10. The detector of claim 9 and further including meansfor deriving from said output voltage signals A and B a measure of theenergy E of the gamma rays causing said scintillation events.
 11. Thedetector of claim 10 wherein said scintillation crystal is generally ofa right circular cylinder shape.
 12. The system of claim 11 wherein saidscintillation crystal is essentially optically transparent and issurrounded on the outside of its cylindrical surface by a lightabsorptive medium.
 13. The system of claim 11 wherein said scintillationcrystal has a predetermined absorptive function for light as a functionof optical path length and is surrounded on its outside cylindricalsurface by a light absorptive medium.
 14. The system of claim 13 whereinsaid predetermined absorptive function for light is a continuousfunction.
 15. The system of claim 11 wherein said scintillation crystalhas a predetermined absorptive function for light as a function ofoptical path length and is surrounded on its outside cylindrical surfaceby a light reflecting medium.
 16. The system of claim 15 wherein saidpredetermined absorptive function for light is a continuous function.